KR910010130B1 - Process for obtaining a ceric oxide - Google Patents

Abstract

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Description

Method of producing cerium oxide

1 is a diagram showing the change of the specific surface of the second cerium oxide in accordance with the firing temperature.

The present invention relates to a method for producing cerium oxide, and more precisely, to a method for increasing the specific surface of cerium oxide and stabilizing the surface even at high temperatures.

The present invention can provide stabilized cerium oxide even at high specific surfaces and at high temperatures.

Specific surface in the context of the present invention refers to the BET specific surface and the nitrogen content according to ASTM D3663-78 method established based on the Burnauer-Emmett-Teller method listed in The Journal of American Society, 60, 309 (1938). It means to measure by adsorption.

It is well known that cerium oxide can be used as a catalyst or a carrier for the catalyst. As an example, Paul MERIAUDEAU et al. Used a platinum catalyst with cerium oxide as a carrier for the synthesis of methanol from CO + H ₂ (CRAcad. Sc. Paris t. 297-Serie II-471). -1983).

It is also well known that the efficacy of a catalyst generally increases with greater surface area where the catalyst and reactant contact. In order to increase the catalytic efficiency, it is necessary to keep the catalyst in as few particles as possible, that is, the smaller the catalyst particles as possible and the better apart one by one. The basic role of the carrier is therefore to keep the catalyst particles or crystals as dispersed as possible in contact with the reactants.

As the catalyst carrier is used for a long time, the specific surface is reduced due to the adhesion of very fine pores. During this process of adhesion, part of the catalyst is buried in the carrier, making contact with the reactants impossible.

Most of the cerium oxide produced up to now shows a rapid decrease in the specific surface at the operating temperature of more than 500 ℃. Therefore, the specific surface of dicerium oxide obtained by calcining R. ALVERO et al. (J. Chem. Soc. Dalton Trans. 1984. 87) from ammonium dinitrate dicerium at 600 ° C. was 29 m ² / g.

On the other hand, the second cerium oxide described in FR-A 2 559 754 was obtained by calcining between 350 ° C and 450 ° C and had a specific surface of at least 85 ± 5 m ² / g, and in particular, by calcining between 400 ° and 450 ° C. Was between 100 and 130 m ² / g. The oxide is obtained by separating the precipitate obtained through hydrolysis of the aqueous solution of cerium oxide in nitric acid solution, washing with an organic solvent, drying and calcining. The cerium oxide thus obtained exhibited an interesting specific surface when obtained by sintering at a temperature of 300 ° C to 600 ° C.

It should be noted that after firing at higher temperatures, the specific surface decreases rapidly. The specific surface of the cerium oxide calcined at 800 ° C. was 10 m ² / g.

In addition, the second cerium oxide recorded in FR-A 2 559 755 exhibited a specific surface of at least 85 ± 5 m ² / g after firing between 350 ° and 500 ° C., but especially after firing between 400 and 450 ° C. The specific surface was between 150 and 180 m ² / g. The oxide is separated from an aqueous solution containing cerium oxide and sulfate ions to precipitate basic cerium sulfate, and the precipitate is separated, washed with ammonia solution, dried and calcined between 300 and 500 ° C. Obtained. The cerium oxide thus obtained has a large specific surface, but the firing operation at 800 ° C. significantly reduces the specific surface to about 10 m ² / g.

One of the objects of the present invention is to provide a method of calcining cerium hydroxide to obtain a cerium oxide having a large specific surface.

Another object of the present invention is to provide a method for improving the stability of the specific surface even at high temperatures.

In order to achieve the first object, the present invention proposes a method for producing the second cerium oxide obtained by calcining the second cerium hydroxide, characterized in that solvothermal traitement of the cerium hydroxide before the firing operation. . In more detail, the method of the present invention consists of:

-Cerium hydroxide is suspended in the liquid medium.

It is heated in a closed container to a temperature and pressure lower than the critical temperature and the critical pressure of the medium.

-Cool the reaction medium and return the pressure to atmospheric pressure.

Remove the treated cerium hydroxide.

-Then fire.

Applicants have found that carrying out a preheating process, termed ″ Autoclavage ″ of cerium hydroxide, before the firing operation is effective in increasing the specific surface of the obtained cerium oxide.

Although the following theoretical description does not in any case limit the importance of the present invention, the autoclave treatment of cerium hydroxide causes recrystallization of hydroxide, which serves to increase the crystallization rate, and consequently reduces the amorphous portion, thereby It is thought that the agglomeration of the crystals between them during the firing operation increases the specific surface of the resulting cerium oxide.

It should be noted that the firing process results in an increase in the size of the crystal, but by limiting this increase through autoclave treatment, it gives the best stability to the specific surface of the crystal.

It is contemplated that the specific surface used in the process of the present invention is probably cerium hydroxide (CeO ₂ .2H ₂ O) or dicerium hydroxide containing a significant amount of bound or adsorbed anions. Thus, the cerium hydroxide used in the process of the present invention may conform to the following general formula (I):

Wherein -A represents an anionic moiety, -a is an integer representing the charge of the anion, -y is a number between 0 and 2, and -n represents a number between 0 and about 20.

In particular the second cerium hydroxide used according to the invention corresponds to the general formula (I) in which a is an integer less than 3 or 3, preferably 1 or 2 and y is a number between 0 and 1.5.

Anionic residue A is chloride, sulphate, nitrate, acetate, formate and the like, preferably nitrate or chloride.

It is also to be noted that in the process of the present invention it is possible to use a second cerium hydroxide which can contain a second cerium oxide phase. Thus, although the cerium compounds described in European Patent Application No. 87 400 600.0 can be used, these cerium compounds cannot be maximally benefited from the advantages obtained by the present invention from the fact that the cerium oxide is present in advance in the autoclave treatment. Neither is it desirable.

Hydroxide of formula (I) prepared according to the process described below consisting of reacting a solution of a cerium salt and a base, optionally in the presence of an oxidant, separating the obtained precipitate and optionally washing and / or drying it. The second cerium forms the preferred starting material.

The solution of cerium salts used may be all aqueous solutions of cerium salts in the first or second cerium state which can be dissolved in the preparation conditions, in particular the first cerium chloride or second cerium state or mixtures thereof.

The cerium salt solution is chosen to contain no impurities that can be recovered from the calcined product. It would be better to use cerium salts that exhibit 99% or more purity.

The concentration of cerium salt solution is not critical in the present invention and its concentration may vary widely but a concentration between 0.2 and 4 mol / l is preferred.

Preferably, it can be transformed into cerium in the first cerium state and oxidized to the second cerium state using an oxidant. Suitable oxidizing agents include perchloric acid, chloric acid, hypochlorous acid, persulfates of sodium, potassium or ammonium, air, oxygen, ozone and the like, in particular the use of hydrogen peroxide. The ratio of oxidant to the first cerium salt to be oxidized can vary widely. Generally, it is better to add an excess of stoichiometry, preferably about 10-40%.

Another preferred method of the invention is to use an aqueous solution of cerium nitrate. The salts generally exhibit a constant initial acidity and may have a normal concentration between 0.1N and ˜5N. The hydrogen ion concentration is not critical but is suitable between 0.1N and 1N.

As a starting material, the second cerium nitrate obtained by reacting nitric acid with hydrated dicerium oxide can also be used. Hydrated cerium oxide can be obtained by the classical method, for example by reacting nitric acid with first cerium carbonate and adding ammonia water in the presence of an oxidizing agent, preferably hydrogen peroxide.

The second cerium nitrate solution obtained according to the electrolytic oxidation method of the first cerium nitrate solution described in French patent application FR-A 2 570 087 (heading 84 13641) may be selected as a starting material.

Precipitation of the second cerium hydroxide can be obtained by the reaction of a cerium salt solution and a basic solution.

The basic solution used is an aqueous solution of ammonia, sodium hydroxide or potassium hydroxide and the like. Preference is given to using ammonia solution. The normal concentration of the basic solution is not important in the present invention and can vary widely, but between 1N and 5N, preferably 2 to 3N is very advantageous.

The ratio of the basic solution to cerium salt solution should be equal to or greater than the equivalent number of cerium. In practice it is preferred to use 5% excess of basic equivalents. The pH of the reaction medium can be between 6 and 10. In particular, it is desirable to adjust the pH constant in units of ± 0.1.

The temperature of the reaction medium is preferably between 5 ° and 95 ° C, more preferably between 40 ° and 70 ° C. The reaction time of the mixture in the reaction medium in the present invention is not critical and can vary widely, but is generally selected between 15 minutes and 2 hours.

Precipitates are obtained using classical solid-liquid separation techniques such as decantation, compaction, filtration and / or centrifugation methods.

It is preferable to remove the residual anions adsorbed on the precipitate by washing the precipitate separated in accordance with the invention. Washing is preferably performed with water or a basic solution having a concentration between 1N and 5N. It is particularly preferable to use ammonia solution. In fact, they are washed several times, most often from one to three times.

Separated and optionally washed dicerium hydroxide can be used directly in the process of the invention. It is also possible to carry out a drying process. Drying can be performed under atmospheric pressure or reduced pressure, for example, under pressure between 1 to 100 mmHg (133.322 Pa to 1332.2 Pa). The drying temperature can be varied from ambient to 100 ° C. and the drying time is not critical but can be between 2 and 48 hours.

The second cerium hydroxide obtained according to the method described below is preferably used in the method for the purpose of the present invention.

According to the method of the present invention, cerium hydroxide is used in the form of a suspension in a liquid medium.

The liquid medium used may be any liquid that does not react at all with the second cerium hydroxide at the temperature and pressure conditions in the present invention.

Among the liquids that can be used, water or basic solutions, in particular hydroxides of alkali metals, in particular sodium hydroxide solutions or all base solutions degradable under the firing conditions of the invention are used. Degradable base means a compound having a pkb lower than 7 and degradable under the firing conditions of the present invention.

Examples of such compounds are ammonia, urea, ammonium acetate, ammonium bicarbonate, ammonium carbonate, or primary, secondary, tertiary-amines such as methylamine, ethylamine, propylamine, n-butylamine, sec-butyl Amine, n-pentylamine, 2-amino-pentane, 2-amino-2-methylbutane, 1-amino-3-methylbutane, 1, 2-diaminoethane, 1, 2-diaminopropane, 1, 3-diaminopropane, 1, 4-diaminobutane, 1, 5-diaminopentane, 1, 6-diaminohexane dimethylamine, diethylamine, trimethylamine, triethylamine or tetramethylammonium hydroxide, And quaternary amines such as tetraalkylammonium hydroxides containing alkyl groups having 1 to 4 carbon atoms such as tetraethylammonium hydroxides. It is also possible to use mixtures of bases.

A preferred variant of the present invention is the use of a solution of degradable bases. This is because surprisingly, in this medium, not only the specific surface of the cerium oxide obtained as a result of the autoclave treatment operation was increased but also the specific surface and the void volume were preserved even at a high temperature up to 900 ° C.

Preference is given to using solutions of ammonia, tetraalkylammonium hydroxides or mixtures thereof.

When the liquid medium is a basic solution, its concentration is not critical to the present invention. The concentration of the solution can vary in a wide range, for example between 0.1 and 11N, but it is usually preferred to use a solution with a concentration between 1 and 10N.

The concentration of the second cerium hydroxide represented by CeO ₂ in the liquid medium can vary between 0.3-6 mole / l, preferably 2-3 mole / l.

The operation of the autoclave is carried out in the range of reflux to critical temperature of the reactants. A temperature between 100 and 350 ° C. may be chosen but preferably between 150 and 350 ° C. is best.

The speed of raising the temperature is not important. The reaction temperature is allowed to reach, for example, by heating for 30 minutes to 4 hours.

The process of the invention can be carried out by introducing a second cerium hydroxide suspended in a liquid medium in an airtight container, the pressure not being due to heating the reaction mixture.

In the above temperature conditions and aqueous media, the pressure is between 1 (10 ⁵ Pa) to 165 (165 · 10 ⁵ Pa) bar, preferably between 5 (5 · 10 ⁵ Pa) to 165 (165 · 10 ⁵ Pa) bar Change. It is also possible to obtain additional external pressure by continuing heating. The operating time of the autoclave is not critical. The time can be between 30 minutes and 6 hours. At the end of this process, the reaction system is left to cool, and the pressure is reduced to atmospheric pressure.

Reaction products suspended in the liquid medium are separated using classical solid-liquid separation techniques such as decantation, compression, filtration and / or centrifugation. The product obtained is washed and / or dried under the conditions described above.

The product obtained according to the last procedure of the process of the invention is calcined at a temperature between 300 and 1000 ° C., preferably between 350 and 800 ° C. The firing time is between 30 minutes and 10 hours, preferably 2 to 6 hours.

By carrying out the process according to the preferential conditions defined below it is possible to obtain oxides with new morphological properties. A feature of the second cerium oxide obtained in the present invention is that it exhibits a specific surface measurement of at least 15 m ² / g after firing between 800 and 900 ° C. The cerium oxide obtained in the present invention exhibits a specific surface measurement of 20 to 60 m ² / g after firing at 800 ° C. According to the firing conditions of the cerium hydroxide, the cerium oxide is calcined at a temperature between 350 ° C and 900 ° C, and then shows a specific surface measurement of 15 to 160 m ² / g.

1 shows a graph of the change (m ² / g) curve (A) of the specific surface of the cerium oxide of the present invention at a calcination temperature ° C.

The second cerium oxide of the present invention exhibits a specific surface measurement of at least 15 m ² / g after firing at a temperature between 800 ° C. and 900 ° C., and shows a higher specific surface when firing at a temperature lower than the aforementioned range. When fired at 350-450 degreeC, the specific surface measurement of 70-160 m <^2> / g, Preferably it is 100-160 m <^2> / g is shown. Nevertheless, when used as a catalyst, at least 15 m ² / g maintains the specific surface even at 900 ℃ or more, preferably exhibits a property of maintaining a specific surface of 20 ~ 60 m ² / g when used at 800 ℃.

The specific surfaces indicated in the present application are those measured on the product after a minimum of two hours firing.

Other properties of the oxide ceric of the present invention is not less than 0.1cm ³ / g measured at a temperature of 800 ° ~ 900 ℃, preferably it shows a pore volume of at least 0.15cm ³ / g.

The pore volume corresponding to a pore of 60 nm (600 mm) or less in diameter is measured by a mercury porosimetry (porosimetre a mercure) according to the standard of ASTM-D 4284-83 or by the nitrogen adsorption isotherm method described above.

As with the specific surface, the void volume also changes with the firing temperature. The void volume is between 0.35 and 0.15 cm ³ / g at a firing temperature between 350 and 900 ° C.

In the present invention, when the cerium oxide is calcined at 800 ° C., a pore volume of 0.15 to 0.25 cm ³ / g is obtained. The pore size of the second cerium oxide calcined at 800 ° C. is between 3 nm (30 μs) and 60 nm (600 μs), and the average diameter of the pores (d ₅₀ ) is 20 nm (200 μs) to 30 nm (300 μs), preferably 25 nm (250 μs). to be. The average diameter is defined as the diameter when the volume of all pores smaller than this diameter occupies 50% of the total pore volume (V _P ) of the pores having a diameter smaller than 60 nm (600 mm ₃ ). The cerium oxide calcined at 350 ° C. exhibits a gap of 20 nm (20 mm 3) to 100 nm (1000 mm 3) and its average diameter is between 10 nm (100 mm 3) and 20 nm (200 mm 3), preferably about 15 nm (150 mm).

X-ray diffraction analysis showed that the cerium oxide of the present invention showed CeO ₂ type single phase crystals having a lattice spacing of 0.542 nm (5.42 kV) to 0.544 nm (5.44 kV). The crystal size of the cerium oxide obtained by calcining at 350 ° C is between 4 nm (40 kPa) and 6 nm (60 kPa), and when calcined at 800 ° C is between 10 nm (100 kPa) and 20 nm (200 kPa). The process for obtaining dicerium oxide having a specific surface of at least 15 m ² / g in the measurement temperature range of 800 ° to 900 ° C. can be characterized by the following facts.

In the presence of an oxidizing agent, cerium salts are reacted with a basic solution (the amount of base so that the pH of the reaction medium is at least 7) to prepare dicerium hydroxide, the precipitate obtained is separated and optionally washed:

This cerium hydroxide is suspended in water or a degradable basic aqueous solution;

In a sealed container they are heated to a temperature and pressure below the critical temperature and the critical pressure of the medium described above;

-Cool the reaction mixture and lower to atmospheric pressure;

Separating the treated second cerium hydroxide;

-Fire.

The reactants used to prepare the second cerium oxide are as defined above. The cerium hydroxide used preferably corresponds to the following molecular formula (II).

[Wherein X represents a chloride ion or a nitrate ion, y is a number less than or equal to 0.5, X is 4-y, and n is a number from 0 to 20.] In a preferred compound corresponding to molecular formula (II), y Is between 0 and 0.1. In addition, x is preferably an anion nitrate.

Compounds such as molecular formula (II) are prepared by reacting a solution of cerium or first cerium nitrate with an ammonia solution in the presence of hydrogen peroxide and the precipitate obtained is separated and washed at least once, preferably with water.

The proportion of the alkali solution to be added is such that the pH of the reactants is 7 or more, preferably between 7.5 and 9.0. The temperature of the reaction medium is chosen between 5 and 70 ° C, preferably between 40 and 70 ° C.

Next, the obtained cerium hydroxide is subjected to autoclave treatment in accordance with the above conditions; The reaction medium of cerium hydroxide is a suspension of a degradable base solution such as ammonia solution or tetraalkylammonium hydroxide solution. Subsequently, the separation, washing and firing processes are carried out as mentioned above.

The cerium oxide according to the present invention has a large specific surface even at a temperature sufficiently high for use in the field of catalysts such as catalysts or catalyst carriers. This compound is particularly suitable for use as a catalyst carrier for the exhaust gas treatment of internal combustion engines.

The following examples illustrate the invention but in no way limit it. Examples 1 to 22 provide evidence for the effect of the autoclave treatment of cerium hydroxide on the specific surface of the cerium oxide obtained by calcining the cerium hydroxide treated according to the present invention. Experiments A through H are presented as comparisons. These are experiments that do not involve autoclave treatment.

Example 1

[Experiment A]

1. Synthesis of Cerium Hydroxide

To a double-walled reactor of 2ℓ capacity stirrer and the reaction was introduced up the device (liquid amount based pumps), and nitric acid the first cerium 922cm ³ eseo 20 ℃ (CeO ₂ containing 179g / ℓ), and the hydrogen peroxide solution to 200 times mukhin solution 38cm ³ at the same time Add for an hour. At the same time, 860 cm ³ of 3N aqueous ammonia solution is added to pH 7 for 1 hour using an injector equipped with a pH controller. After all the reactants were added, the temperature of the reaction medium was maintained at 70 ° C. for one hour. Next, the sediment is separated with a Buchner funnel. Here, 280 g of dicerium hydroxide having a molar ratio of NO ₃ ⁻ / Ce of 0.16 is obtained. X-ray diffraction analysis showed that this cerium hydroxide was a crystal with an average diameter of 5.5 nm (55 kV).

2. Autoclave Treatment of Cerium Hydroxide

30 cm ³ of deionized water and 30 g of the prepared cerium hydroxide were sequentially added to a high 100 cm ³ beaker. They are mixed uniformly in the medium and the beaker is placed in an autoclave of about 0.5 L capacity. The reaction system is allowed to react with moderate heating at 200 ° C. at about 16 bar (16.10 ⁵ Pa) for about 4 hours.

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. X-ray diffraction analysis of the wet reactant determined that the average diameter of the crystal was 6.5 nm (65 mm 3). The baking operation is then carried out at 350 ° C. for 2 hours.

The specific surface of the second cerium oxide obtained according to the above-defined method and the pore volume of 60 nm (600 Pa) or less were measured. At the same time, X-ray diffraction analysis is performed to determine the crystal size perpendicular to the 110 and 220 planes. The results obtained are shown in Table I. For comparison, the results of the second cerium oxide prepared by calcining the second cerium hydroxide synthesized in Experiment 1 1 at 350 ° C. for 2 hours are also shown.

TABLE I

It can be seen that the specific surface of the second cerium oxide obtained by autoclave treatment and calcining at 350 ° C. is more than two times larger and shows smaller crystal size.

Example 2

[Experiment B]

1. Synthesis of Cerium Hydroxide

In a device described in Example 1, at 950 g, 179 g / L of a first cerium nitrate solution containing CeO ₂ and 922 cm ³ of 200 times diluted hydrogen peroxide solution 38 cm ³ were charged. 4.6 cm of aqueous ammonia 860 cm ³ is added until the pH reaches 9.0. After adding all the reactants, the reaction medium is maintained at 70 ° C. for 1 hour. The sediment is then separated with a Buchner funnel. To give the / Ce hydroxide ceric 515g molar ratio of 0.1 was of ^- NO _3. X-ray diffraction analysis showed that this cerium hydroxide was a crystal having an average diameter of 3 nm (30 μs).

2. Autoclave Treatment of Cerium Hydroxide

According to the operation described in Example 1, 30 g of the prepared cerium hydroxide was suspended in 30 cm ³ of deionized water and subjected to autoclave treatment at 200 ° C. for 4 hours.

X-ray diffraction analysis of the wet product determined that the crystal had an average diameter of 4.5 nm (45 mm 3). After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. The baking operation is then performed at 350 ° C. for 2 hours.

Specific Surface of Second Cerium Oxide Crystals Prepared by Directly Calcining Cerium Oxide (Example 2) and Secondary Cerium Hydroxide Synthesized in Experiment 1 B for Two Hours at 350 ° C. for Comparison The pore volume and crystal size are measured. The results obtained are shown in Table II.

TABLE II

As in Example 1, it can be seen that the autoclave treatment has a desirable effect on the specific surface and void volume of CeO ₂ obtained by firing at 350 ° C.

Example 3

[Experiment C]

1. Synthesis of Cerium Hydroxide

In a device such as that described in Example 1, at 50 ° C., 179 g / L of a first cerium nitrate solution containing CeO ₂ and 922 cm ³ of 200 times diluted hydrogen peroxide solution 38 cm ³ are charged.

4N aqueous ammonia solution 825cm ³ is added until the pH is 8.4.

After all the reactants were added, the reaction medium was maintained at 70 ° C. for 1 hour.

The sediment is then separated with a Buchner funnel. To give the / Ce hydroxide ceric 569g molar ratio of 0.1 was of ^- NO _3. As a result of X-ray diffraction analysis, it can be seen that the second cerium hydroxide is a crystal having 3.0 nm (30 μs).

2. Autoclave Treatment of Cerium Hydroxide

According to the operation described in Example 1, 30 g of the prepared cerium hydroxide was suspended in 30 cm ³ of deionized water and subjected to autoclave treatment at 200 ° C. for 4 hours.

X-ray diffraction analysis of the wet product determined that the crystal had an average diameter of 4.5 nm (45 mm 3).

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. The baking operation is then performed at 350 ° C. for 2 hours.

Specific Surface of Second Cerium Oxide Crystals Prepared by Autoclave Treatment of Cerium Oxide (Example 3) and Secondary Cerium Hydroxide Synthesized in Experiment C of Example C for Direct Comparison at 350 ° C. for 2 Hours The pore volume and crystal size are measured. The results obtained are shown in Table III.

TABLE III

It can be seen that the second cerium oxide obtained by firing at 350 ° C. has a larger specific surface, pore volume, and smaller crystal size.

Example 4

[Experiment D]

1. Synthesis of Cerium Hydroxide

In a device such as that described in Example 1, at 50 ° C., 179 g / L of a first cerium nitrate solution containing CeO ₂ and 922 cm ³ of 200 times diluted hydrogen peroxide solution 38 cm ³ are charged.

4N ammonia water 825cm ³ is added until the pH is 8.4.

After adding all the reagents, the reaction medium is maintained at 70 ° C. for 1 hour.

The sediment is then separated with a Buchner funnel. X-ray diffraction analysis showed that this cerium hydroxide was a crystal having an average diameter of 3.0 nm (30 μs).

2. Autoclave Treatment of Cerium Hydroxide

According to the operation described in Example 1, 30 g of the prepared cerium hydroxide was suspended in 30 cm ³ of deionized water and subjected to autoclave treatment at 200 ° C. for 4 hours.

X-ray diffraction analysis of the wet product determined that the crystal had an average diameter of 4.5 nm (45 mm 3).

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. Then, the baking operation is performed at 800 ° C. for 2 hours. Specific surface of the second cerium oxide crystals prepared by autoclaving crystals of cerium oxide (Example 4) and the second cerium hydroxide synthesized in Experiment 1 for direct comparison at 800 ° C. for 2 hours, Measure the void volume and crystal size.

The results obtained are shown in Table IV.

TABLE IV

Compared to Example 3, in Example 4 the cerium oxide is calcined at higher temperatures. As the firing temperature increases from 350 ° C. to 800 ° C., the crystal size increases, and thus the specific surface and pore volume of the crystal decrease.

Example 5

[Experiment C]

1. Synthesis of Cerium Hydroxide

It is the same as the operation described in Example 3-1.

2. Autoclave Treatment of Cerium Hydroxide

According to the operation described in Example 1, 30 g of the prepared cerium hydroxide was suspended in 30 cm ³ of a 1N aqueous ammonia solution and subjected to autoclave treatment at 200 ° C. for 4 hours.

X-ray diffraction analysis of the wet product determined that the crystal had an average diameter of 4.0 nm (40 mm 3).

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. The baking operation is then performed at 350 ° C. for 2 hours. Specific Surface of Second Cerium Oxide Crystals Prepared by Autoclave Crystal of Cerium Oxide (Example 5) and Secondary Cerium Hydroxide Synthesized in Experiment C of Example C for Direct Comparison at 350 ° C. for 2 Hours The pore volume and crystal size are measured.

The results obtained are shown in Table V.

TABLE V

As in the above examples, it can be seen that the specific surface and the pore volume of the second cerium oxide obtained by calcining at 800 ° C. have a desirable effect of autoclave treatment.

Example 6

[Experiment D]

1. Synthesis of Cerium Hydroxide

The same operation as that described in Example 4-1 is performed.

2. Autoclave Treatment of Cerium Hydroxide

According to the operation described in Example 1, 30 g of the prepared cerium hydroxide was suspended in 30 cm ³ of a 1N aqueous ammonia solution and subjected to autoclave treatment at 200 ° C. for 4 hours.

X-ray diffraction analysis of the wet product determined that the crystal had an average diameter of 4.0 nm (40 mm 3).

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel.

Then, the baking operation is performed at 800 ° C. for 2 hours. Specific Surface of Second Cerium Oxide Crystals Prepared by Autoclave Treatment of Cerium Oxide (Example 6) and Secondary Cerium Hydroxide Synthesized in Experiment 1 for Direct Comparison at 800 ° C. for 2 Hours The pore volume and crystal size are measured.

The results obtained are shown in Table VI.

Table VI

It can be seen that the autoclave treatment of cerium hydroxide in ammonia medium prevents the crystal size of the cerium oxide obtained by calcining at 800 ° C. from exceeding 20 nm (200 kPa).

EXAMPLES 7 AND 8

1. Synthesis of Cerium Hydroxide

The same operation as described in Example 3-1 is performed.

2. Autoclave Treatment of Cerium Hydroxide

Into a high 100 cm ³ beaker, 25 cm ^{3 of an} aqueous 20% tetraethylammonium hydroxide solution and 15 g of the prepared cerium hydroxide were sequentially added.

After the reaction medium is uniformly mixed, the beaker is placed in an autoclave. The reaction vessel is appropriately heated at 200 ° C. at about 16 bar (16.10 ⁵ Pa) for about 3 hours.

After this heat treatment the precipitate is filtered with a Buchner funnel. The following calcination conditions: in Example 7, the firing operation was carried out at 350 ° C. for 2 hours, in Example 8 at 800 ° C. for 1 hour. Then, the specific surface and pore volume of the obtained cerium oxide are measured.

The results are shown in Table VII.

TABLE VII

[Examples 9-20]

[Experiment E and F]

1. Synthesis of Cerium Hydroxide

Add the embodiment example 1 by nitric acid may first a hydrogen peroxide solution diluted cerium 922cm ³ times with 200 containing CeO ₂ of 150g / ℓd at ambient temperature in the apparatus as described in the solution of 30cm ^3.

150 cm ³ of 3N ammonia solution is added while maintaining 8 ° C. until pH 9.5 is obtained. After adding all the reactants, the reaction medium was maintained at 8 ° C. for 1 hour.

The precipitate is separated using a Buchner funnel and washed with water.

X-ray diffraction analysis shows that the cerium hydroxide is a crystal having an average diameter of 3.5 nm (35 mm 3).

2. Autoclave Treatment of Cerium Hydroxide

A series of experiments is performed while varying the temperature of the autoclave between 160 ° C and 330 ° C.

According to the method described in the above embodiments, 150 g of the prepared cerium hydroxide was suspended in 150 cm ³ of an aqueous 1N ammonia solution and subjected to autoclave treatment for 4 hours.

After this heat treatment the precipitate is filtered with a Buchner funnel. The following calcination conditions: firing operation under 350 ° C. for 2 hours in Examples 9 to 14, 800 ° C. for 2 hours in Examples 15 to 20. The specific surface and the pore volume of the obtained cerium oxide are measured.

For comparison, the cerium hydroxide synthesized in 1 is directly calcined for 2 hours at 350 ° C. (Experiment E) and 800 ° C. (Experiment F) to give the result obtained by the cerium oxide produced.

All results obtained are shown in Table VIII.

TABLE VIII

Example 21

[Experiment G]

1. Synthesis of Cerium Hydroxide

A 2-liter double wall reactor with a water temperature controlled to 20 ° C. is installed and a stirring device and a reactant injection device are continuously injected simultaneously with the following reagents.

Aqueous solution of cerium nitrate containing 0.5N of cerium IV and 0.06mole / L of cerium III and having a free acidity of 0.5 N prepared by electrolysis based on FR-2 570 087 (n ° 8413641) Inject at a rate of 0.92 l / hr.

A -3N aqueous ammonia solution is injected at a rate of 1.08 l / hr. The resulting mixture is stirred for 60 minutes at a stirring speed of 300 times / minute. The amount of solution added is adjusted to maintain a pH of 5.5. The precipitate formed is separated by filtration with a Buchner funnel. A product containing 42% CeO _{2 and} having a crystal size smaller than 3 nm (30 mm 3) is obtained.

2. Autoclave Treatment of Cerium Hydroxide

A 0.5 L autoclave 300 cm ³ of 1N NH ₄ OH solution was added followed by 100 g of the obtained cerium hydroxide.

After the mixture is uniformly mixed in the medium, the reaction vessel is appropriately heated at 200 ° C. at 16 bar (16.10 ⁵ Pa) for 3 hours.

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. The firing operation is carried out at 350 ° C. for 3 hours and at 800 ° C. for 1 hour.

According to the method defined above, the specific surface and the pore volume of the obtained cerium oxide are measured.

The results obtained are shown in Table IX. For comparison, the measurement results of the second cerium oxide obtained by firing at 350 ° C. for 2 hours and at 800 ° C. for 1 hour are also shown.

TABLE IX

Example 22

[Experiment H]

1. Synthesis of Cerium Hydroxide

A 2-liter double wall reactor with a water temperature controlled to 20 ° C. is installed and a stirring device and a reactant injection device are continuously injected simultaneously with the following reagents.

Dicerium nitrate with 0.5N free acidity, prepared by electrolysis based on FR-A 2 570 087 (n ° 84 13641), containing -1 mole / l cerium IV and 0.06 mole / l cerium III The aqueous solution is injected at a rate of 0.78 L / hr.

Aqueous solution of -3N ammonia is injected at a rate of 1.22 l / hr. The resulting mixture is stirred for 60 minutes at a stirring speed of 300 times / minute.

The unit hourly dose of these solutions is adjusted to maintain the pH at 9.0.

The precipitate formed is separated by filtration with a Buchner funnel. A product containing 20% by weight of cerium oxide and having a crystal size smaller than 3 nm (30 mm 3) is obtained.

2. Autoclave Treatment of Cerium Hydroxide

A 0.5 L autoclave 300 cm ³ of 1N NH ₄ OH solution was added followed by 100 g of the obtained cerium hydroxide. After the mixture is uniformly mixed in the medium, the reaction vessel is appropriately heated at 200 ° C. at about 16 bar (16.10 ⁵ Pa) for 3 hours.

After this hydrothermal treatment the precipitate is filtered with a Buchner funnel. The firing operation is carried out at 350 ° C. for 2 hours and at 800 ° C. for 1 hour.

According to the method defined above, the specific surface and the pore volume of the obtained cerium oxide are measured. The measurement results are shown in Table 10.

Also shown are the measurement results of the second cerium oxide obtained by calcining the second cerium hydroxide synthesized in 1 for two hours at 350 ° C. and the second cerium oxide obtained by calcining directly at 800 ° C. for 1 hour.

TABLE X

It can be seen that the autoclave treatment has a favorable effect on the specific surface and pore volume of the first cerium oxide obtained by firing at 800 ° C.

Claims (34)

A method for producing cerium oxide obtained by calcining cerium hydroxide, characterized in that a soluble solvent heat treatment is carried out before the calcining operation. The method of claim 1, wherein the method comprises:-suspending cerium hydroxide in a liquid medium; Heating in a hermetically sealed container to a temperature and pressure below the critical temperature and below the critical pressure of the medium; Cooling the reaction medium, lowering the pressure to atmospheric pressure, and separating the treated cerium hydroxide; And then calcining it. The method for producing cerium oxide according to claim 2, wherein the cerium hydroxide is in accordance with the following general formula (I).

[Wherein A represents an anionic residue, a is an integer representing the charge of the anion, y is a number between 0 and 2, and n is a number between 0 and about 20]. 4. A process for producing cerium oxide according to claim 3, wherein the cerium hydroxide corresponds to formula (I) wherein a is 3 or less in formula. The method for producing cerium oxide according to claim 3 or 4, wherein the cerium hydroxide corresponds to general formula (I) in which a is 1 or 2. 4. A process for producing cerium oxide according to claim 3, wherein the cerium hydroxide corresponds to general formula (I) in which y is between 0 and 1.5. The method of claim 3, wherein the anionic moiety is chloride, sulfate, nitrate, acetate, formate. 8. The method of claim 7, wherein the anionic moiety is chloride or nitrate. 4. A process according to claim 3, characterized in that the cerium hydroxide is prepared by reacting a solution and a base of cerium salt, optionally in the presence of an oxidizing agent, separating the precipitate obtained and optionally washing and / or drying it. Method for producing cerium oxide. 10. The method of claim 9, wherein the solution of cerium salt is a solution of first cerium chloride, or a nitrate in the first cerium or second cerium state, or a mixture thereof. The method for producing cerium oxide according to claim 10, wherein the solution of cerium salt is an aqueous solution of cerium nitrate or cerium nitrate used in the presence of an oxidizing agent. 12. The cerium oxide of claim 11, wherein the solution of cerium salt is an aqueous solution of cerium oxide calculated by electrochemically oxidizing a solution of first cerium nitrate or by acting nitric acid on hydrous second cerium oxide. Manufacturing method. 10. The process of claim 9, wherein the base is an ammonia solution. 10. The method of claim 9, wherein the oxidant is hydrogen peroxide solution. 10. The method of claim 9, wherein the amount of base added is such that the pH of the reaction medium is from about 6 to about 10. 16. The method of claim 15, wherein the pH is 7.0-9.0. 10. The method of claim 9, wherein the temperature of the reaction medium is 5 ° C to 95 ° C. 18. The process for producing cerium oxide according to claim 17, wherein the temperature of the reaction medium is 40 ° C to 70 ° C. 10. The method of claim 9, wherein the separated cerium hydroxide is washed with water or a basic solution. The process for producing cerium oxide according to claim 1, wherein the liquid medium is water or a basic solution. 21. The method of claim 20, wherein the basic solution is a solution of an alkali metal hydroxide or a solution of a base decomposable under calcination conditions. 22. The method of claim 21, wherein the degradable base is ammonia, urea, ammonium bicarbonate, ammonium carbonate, primary, secondary, tertiary or quaternary amine, or mixtures thereof. 23. The method of claim 22, wherein the degradable base is ammonia, tetraalkyl ammonium hydroxide or mixtures thereof. The method for producing cerium oxide according to any one of claims 20 to 23, wherein the concentration of the basic solution is between 1 and 10 N. The method for producing cerium oxide according to claim 2, wherein the concentration of the second cerium hydroxide represented by CeO ₂ is 0.3 to 6 mol / l. The method of claim 25, wherein the concentration is 2-3 mol / l. The method for producing cerium oxide according to claim 2, wherein the autoclave treatment temperature is 100 to 350 ° C. The method of claim 27, wherein the temperature is 150 ~ 350 ℃. 3. The method of claim 2, wherein the pressure is between 1 bar (10 ⁵ Pa) and 165 bar (165 x 10 ⁵ Pa). 30. The method of claim 29, wherein the pressure is between 5 bar (5 x 10 ⁵ Pa) and 165 bar (165 x 10 ⁵ Pa). 31. The process for producing cerium oxide according to any one of claims 27 to 30, wherein the autoclave treatment time is 30 minutes to 6 hours. The method for producing cerium oxide according to claim 2, wherein the firing temperature is 300 to 1000 ° C. The method of claim 32, wherein the temperature is 350 ~ 800 ℃. 34. The method for producing cerium oxide according to claim 32 or 33, wherein the firing time is 2 to 6 hours.

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